Showing papers on "Exciton published in 1995"

Efficient photodiodes from interpenetrating polymer networks

Abstract: THE photovoltaic effect involves the production of electrons and holes in a semiconductor device under illumination, and their subsequent collection at opposite electrodes. In many inorganic semiconductors, photon absorption produces free electrons and holes directly1. But in molecular semiconductors, absorption creates electrona¤-hole pairs (excitons) which are bound at room temperature2, so that charge collection requires their dissociation. Exciton dissociation is known to be efficient at interfaces between materials with different electron affinities and ionization potentials, where the electron is accepted by the material with larger electron affinity and the hole by the material with lower ionization potential3. A two-layer diode structure can thus be used, in which excitons generated in either layer diffuse towards the interface between the layers. However, the exciton diffusion range is typically at least a factor of 10 smaller than the optical absorption depth, thus limiting the efficiency of charge collection3. Here we show that the interpenetrating network formed from a phase-segregated mixture of two semiconducting polymers provides both the spatially distributed interfaces necessary for efficient charge photo-generation, and the means for separately collecting the electrons and holes. Devices using thin films of these polymer mixtures show promise for large-area photodetectors.

Observation of the "Dark exciton" in CdSe quantum dots.

Abstract: We use external magnetic fields to identify the band edge emitting state in CdSe quantum dots. The field dependence of emission decays and LO phonon spectra show the importance of exciton spin dynamics in the recombination mechanism. To interpret our results we calculate the band edge exciton structure, including the effects of the electron-hole exchange interaction and a nonspherical shape. The exchange term, negligible in the bulk, is strongly enhanced by quantum confinement and allows the observation of an optically passive "dark" excitonic state.

Ultrafast coherent control and destruction of excitons in quantum wells.

Abstract: We demonstrate the coherent destruction of photogenerated excitons in semiconductor quantum wells within a few hundred femtoseconds of their excitation. Coherent control of carrier dynamics is achieved with phase-locked pairs of 100 fs infrared pulses. The technique induces an optical response which is faster than the inverse of the exciton linewidth superseding Fourier limits for a single pulse. Energy selectivity enables the coherent transfer of angular momentum between hole states. Such phase-tailored pulse trains can be utilized to investigate the generation process and intermediate virtual states in quantum structures.

Optical spectroscopy of a two-dimensional electron gas near the metal-insulator transition.

Abstract: We report on optical measurements of a two-dimensional electron gas near the metal-insulator transition. We observe the appearance of excitons and negatively charged excitons, ${X}^{\ensuremath{-}}$, at the onset of the transition. The fact that these excitons appear at a relatively large average electron density shows that transition is induced by localization of single electrons in the electrostatic potential fluctuations of the remote ionized donors.

Green luminescence from copper doped zinc sulphide quantum particles

Abstract: Free‐standing powder of zinc sulphide quantum particles has been synthesized using a chemical route. X‐ray diffraction analysis shows that the diameter of the particles is ∼21±2 A which is smaller than the Bohr exciton diameter for zinc sulphide. UV absorption shows an excitonic peak centered at ∼300 nm corresponding to an energy gap of 4.1±0.1 eV. These particles show a luminescence band at ∼424 nm. The quantum particles could be doped with copper during synthesis without altering the UV absorption or x‐ray diffraction pattern. However, doping shifted the luminescence to 480 nm, green wavelength in the visible region.

Superlattices and Other Heterostructures: Symmetry and Optical Phenomena

Abstract: 1 Quantum Wells and Superlattices.- 2 Crystal Symmetry.- 2.1 Symmetry Operations, Groups.- 2.2 Point-Group Classification.- 2.3 Space Groups.- 2.4 Group Representations, Characters.- 2.5 Point-Group Representations.- 2.6 Spinor Representations.- 2.7 Representations of Space Groups.- 2.8Invariance Under Time Inversion.- 2.9 Selection Rules.- 2.10 Determination of Linearly Independent Components of Material Tensors.- 3 Electron Spectrum in Crystals, Quantum Wells and Superlattices.- 3.1 The k-p Method.- 3.2 The Effective-Mass Method Deformation Potential.- 3.3 Method of Invariants.- 3.4 Electron and Hole Spectrum in Diamond-and Zincblende-Type Cubic Crystals.- 3.5 Electron Spectra of Quantum Wells and Superlattices.- 3.6 Hole Spectrum in Quantum Wells and Superlattices for Degenerate Bands.- 3.7 Deformed and Strained Superlattices.- 3.8 Quantum Wells and Superlattices in a Magnetic Field.- 3.9 Spectrum of Quantum Wells and Superlattices in an Electric Field.- 4 Vibrational Spectra of Crystals and Superlattices Electron-Phonon Interaction.- 4.1 Normal Vibrations: Distribution in Irreducible Representations.- 4.2 Vibrational Spectra of Superlattices.- 4.3 Electron-Phonon Interaction.- 5 Localized Electron States and Excitons in Heterostructures.- 5.1 Shallow Impurity Centers.- 5.2 Localized States at Superlattice Defects.- 5.3 Excitons.- 5.4 Exchange Splitting of Exciton Levels.- 6 Interband Optical Transitions.- 6.1 Optical Superlattices.- 6.2 Interband Transitions and Dielectric Susceptibility of a Periodic Heterostructure.- 6.3 Coulomb Interaction Between the Electron and the Hole.- 6.4 Exciton Polaritons in an Optical Superlattice.- 6.5 Light Reflection.- 6.6 Electro-Optical Effects in Interband Transitions.- 6.7 Magneto-Optical Spectra.- 7 ntraband Transitions.- 7.1 Cyclotron Resonance and Effective Electron Mass.- 7.2 Intersubband Absorption.- 7.3 Electron-Spin Resonance.- 7.4 IR Reflection in an Undoped Superlattice.- 8 Light Scattering.- 8.1 Theory of Light Scattering in Semiconductors.- 8.2 Scattering by Intersubband Excitations.- 8.3 Scattering by Acoustical Phonons with a Folded Dispersion Law.- 8.4 Scattering by Optical Phonons in Heterostructures.- 8.5 Acceptor Spin-Flip Raman Scattering.- 9 Polarized Luminescence in Quantum Wells and Superlattices.- 9.1 Luminescence as a Tool to Study Electronic Spectra and Kinetic Processes in Two-Dimensional Systems.- 9.2 Luminescence in the Quantum Hall Regime, Quantum Beats.- 9.3 Optical Spin Orientation and Alignment of Electron Momenta.- 9.4 Optical Orientation and Alignment of Excitons.- 9.5 Polarized Luminescence of Excitons and Impurities in an External Magnetic Field.- 10 Nonlinear Optics.- 10.1 Two-Photon Absorption.- 10.2 Photoreflectance.- 10.3 Diffraction from a Light-Induced Spatial Grating.- 10.4 Third-Harmonic Generation.- 10.5 Linear and Circular Photogalvanic (Photovoltaic) Effects.- 10.6 Current of Optically Oriented Electrons.- 10.7 Photon Drag Current.

Exciton condensate in semiconductor quantum well structures.

Abstract: We propose that the exciton condensate may form in a well-controlled way in appropriately arranged semiconductor quantum well structures. The mean-field theory of Keldysh and Kopaev, exact in both the high density and low density limits, is solved numerically to illustrate our proposal. The electron-hole pairing gap and the excitation spectrum of the exciton condensate are obtained. The energy scales of the condensate are substantial at higher densities. We discuss how such densities could be achieved experimentally by generating an effective pressure.

The dynamics of one‐dimensional excitons in liquids

Abstract: The properties of excitons in one‐dimensional molecular aggregates, dissolved at room temperature in a liquid, were studied by means of femtosecond nonlinear optical experiments. Both the one‐exciton band (i.e., Frenkel‐excitons) and multiexciton bands contribute to the observed nonlinear optical response. The rapid motions in the liquid lead to ultrafast perturbations of the molecular energy levels. This localizes the excitons on limited sections of the chains of aggregated molecules. Ultrafast frequency‐resolved pump–probe spectroscopy on the lowest two exciton bands was employed to determine the delocalization length of the optical excitations. The kinetics of the exciton populations was measured by ultrafast grating scattering experiments and time‐resolved single photon counting. A model is described in which the multiexciton bands act as doorway states in the exciton–exciton annihilation process. These bands thereby determine the population decay of the Frenkel excitons at high excitation densities. Room temperature photon echo experiments show that stochastic perturbations of the exciton transition frequencies occur on two distinct time scales. In particular the slow components of the fluctuations are affected by motional narrowing, associated with the exciton delocalization length. It is therefore argued that the optical dephasing of excitons is directly related to the spatial extent of the excitation on the aggregate chain.

Photoelectric emission from negative-electron-affinity diamond (111) surfaces: Exciton breakup versus conduction-band emission

Abstract: We have recently reported that bound electron-hole pairs (Mott-Wannier excitons) are the dominant source of photoelectron emission from specially prepared [``as-polished'' C(111)-(1\ifmmode\times\else\texttimes\fi{}1):H] negative-electron-affinity diamond surfaces for near-band-gap excitation up to 0.5 eV above threshold [C. Bandis and B. B. Pate, Phys. Rev. Lett. 74, 777 (1995)]. It was found that photoexcited excitons transport to the surface, break up, and emit their electron. In this paper, we extend the study of exciton-derived emission to include partial yield (constant final-state) analysis as well as angular distribution measurements of the photoelectric emission. In addition, we find that exciton-derived emission does not always dominate. Photoelectric emission properties of the in situ ``rehydrogenated'' (111)-(1\ifmmode\times\else\texttimes\fi{}1):H diamond surface are characteristically different than emission observed from the as-polished (111)-(1\ifmmode\times\else\texttimes\fi{}1):H surface. The rehydrogenated surface has additional downward band bending as compared to the as-polished surface. In confirmation of the assignment of photoelectric yield to exciton breakup emission, we find a significant enhancement of the total electron yield when the downward band bending of the hydrogenated surface is increased. The functional form of the observed total electron yield demonstrates that, in contrast to the as-polished surface, conduction-band electrons are a significant component of the observed photoelectric yield from the in situ hydrogenated (111)-(1\ifmmode\times\else\texttimes\fi{}1):H surface. Furthermore, electron emission characteristics of the rehydrogenated surface confirms our assignment of a Fan phonon-cascade mechanism for thermalization of excitons.

Screening in Semiconductor Nanocrystallites and Its Consequences for Porous Silicon.

Abstract: Consequences of the modified dielectric properties of semiconductor crystallites are explored. The notion and usefulness of an effective dielectric constant are analyzed using a self-consistent linear screening calculation. The binding energy of hydrogenic impurities is defined and calculated, and it is shown why these are always ionized in porous silicon. Self-energy terms associated with the surface polarization charge are discussed in the context of Coulomb charging effects. Their contribution to exciton binding energies is also determined. Consequences of charging effects on carrier injection in porous silicon are finally considered and shown to be important.

Exciton Coupling Effects in the Absorption and Photoluminescence of Sexithiophene Derivatives

Abstract: published in Advance ACS Abstracts, May 1, 1995. du Markcha1 Joffre, 92002 Nantene, France. 0022-365419512099-9155$09.00/0 SCHEME 1: Chemical Structure of Sexithiophene Derivatives 6T DH6T ClOH21 ClOH21 DD6T have been widely discussed in the literature, and particularly in the case of sexithiophene (6T), very little work has been carried out on the optical characteristics of 6T, and the energetic diagram of this compound is still an object of debate. From photoluminescence measurements under one- and two-photon excitation on a polycrystalline thin film of 6T, an electronic level ordering has been proposed by Taliani et al. in which it is suggested that the lowest Ag exciton level lies at 898 cm-’ above the lowest one-photon-allowed lB, exciton leveL4 Athouel et al. have reported that the n-n* transition in p-sexiphenyl is affected by orientation of these molecules in a thin film.5 Hamano et al. have fabricated molecular oriented thin films of 6T by organic molecular beam deposition.

Exciton-LO-phonon quantum kinetics: Evidence of memory effects in bulk GaAs.

Abstract: Oscillations of the transient four-wave-mixing signal with a period of about $100\mathrm{fs}$ are observed in bulk GaAs using 14 fs pulses tuned to the exciton resonance at low temperatures. The measurements are explained in terms of the non-Markovian quantum kinetics for electron-hole pairs due to LO-phonon scattering. It is shown that the observed oscillations are evidence for memory effects. The experiments provide a first test of the central ideas of quantum kinetics, in which the effects of quantum coherence and of dissipation are intrinsically connected.

Exciton dynamics in electroluminescent polymers studied by femtosecond time-resolved photoluminescence spectroscopy

Abstract: We report measurements of the time-resolved photoluminescence from unoriented films of poly[2-methoxy,5-(${2}^{\ensuremath{'}}$-ethyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(p-phenylenevinylene) (PPV). An extremely rapid luminescence rise is observed over the entire spectral region measured. A broad high-energy luminescence tail can be seen for very short times after excitation. A redshift of the luminescence with time is also observed and is larger in MEH-PPV than in PPV. The measured decay times of the photoluminescence indicate that the nonradiative decay of singlet excitons is more rapid in MEH-PPV than in PPV, lowering its photoluminescence efficiency. We attribute the differences between the materials to the reduced crystallinity of MEH-PPV compared to PPV. This increases the density of conformational defects and thus reduces the effective conjugation length of the polymer chains. These defects also provide a more efficient nonradiative decay channel. Polarized measurements of the luminescence were performed and provide insight into the ultrafast dynamics of the excitons.

Structure in the lowest absorption feature of CdSe quantum dots

Abstract: We use transient differential absorption experiments to investigate the ‘‘single dot’’ absorption line shape of CdSe quantum dots. We observe both a narrow (full width half maximum ∼5 meV) and a broad (∼50 meV) bleach component within the inhomogeneously broadened first absorption line of our samples. We deduce the single dot absorption line shape which is most consistent with the experimental results. This line shape, which contains structure in the lowest quantum dot absorption feature, explains the large ‘‘Stokes’’ shift seen in the full band edge luminescence of CdSe quantum dots. We discuss the structure within the context of several competing models. The data appear inconsistent with models which use surface states to explain the anomalous emission behavior of II–VI quantum dots. Instead they imply that exciton fine structure is observed in our samples.

Spin-triplet negatively charged excitons in gaas quantum wells

Abstract: We observe magnetic-field-induced transitions in the interband optical spectra of GaAs quantum wells with a small excess electron density. Their strengthening with excess electron density, in addition to their light polarization dependence, demonstrate that these correspond to (excited) spin-triplet states of the negatively charged exciton. The second-electron binding energy of both singlet and triplet ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ strengthens with field.

Optical dispersion by Wannier excitons.

Abstract: An analytical expression of the complex dielectric constant of Wannier excitons is obtained, which includes exactly the contributions of all bound and unbound states. It allows an improved description of the excitonic influence on the optical properties of semiconductors near the band gap, especially with respect to dispersion effects.

Microcavity exciton-polariton splitting in the linear regime.

Abstract: Comparisons are made of a classical and a quantum mechanical model of polariton energy splitting inside a microcavity in the linear excitation regime. Analytical expressions are obtained for the splitting energy as a function of temperature, detuning, and exciton-photon coupling constants. Experimental results are compared with theory, and agreements are found only if the effect of inhomogeneity is taken into account.

Quantum-confined Stark effects in semiconductor quantum dots

Abstract: Quantum-confined Stark effects (QCSE) on excitons, i.e., the influence of a uniform electric field on the confined excitons in semiconductor quantum dots (QD's), have been studied by using a numerical matrix-diagonalization scheme. The energy levels and the wave functions of the ground and several excited states of excitons in CdS and ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Se}}_{\mathit{x}}$ quantum dots as functions of the size of the quantum dot and the applied electric field have been obtained. The electron and hole distributions and wave function overlap inside the QD's have also been calculated for different QD sizes and electric fields. It is found that the electron and hole wave function overlap decreases under an electric field, which implies an increased exciton recombination lifetime due to QCSE. The energy level redshift and the enhancement of the exciton recombination lifetime are due to the polarization of the electron-hole pair under the applied electric field.

Electron Emission Due to Exciton Breakup from Negative Electron Affinity Diamond.

Abstract: We identify exciton breakup at the surface as the dominant source of photoelectron emission from negative electron affinity diamond (111): H for near band gap excitation up to 0.5 eV above threshold. Dependence of photoelectron emission characteristics upon excitation energy suggests the thermalization of carriers via a Fan phonon-cascade mechanism.

Electron-concentration-dependent quantum-well luminescence: Evidence for a negatively charged exciton

Abstract: The quantum-well electroluminescence of p-i-n GaAs/AlAs double-barrier resonant tunneling diodes has been investigated. The bias-dependent tunability of the resonance conditions for electron and hole tunneling allows for a continuous change of the relative electron and hole concentrations in the quantum well. As the electron concentration is increased, the quantum-well emission line due to heavy-hole free exciton recombination is replaced by a new excitonic line, 2 meV lower in energy. This line is attributed to a negatively charged exciton ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$. The magnetic field and temperature dependence of the quantum-well emission have been used to characterize this transition.

Exciton-exciton correlation in the nonlinear optical regime.

Abstract: We present a theory of the time development of the nonlinear optical excitations in a semiconductor in which the correlated part of the many-body problem is expressed in terms of the exact $N$-exciton eigenstates and is decoupled from the optical excitation process. A first-principles exact numerical solution for the four-wave mixing in a one-dimensional model demonstrates the role of two-exciton correlation in polarization mixing and, in particular, shows a ringing of the polarization with the frequency given by the binding energy of the biexciton.

Polaron and exciton-phonon complexes in CuCl nanocrystals.

Abstract: We report the first evidence of an excitonic polaron and exciton-phonon complexes in nanosize semiconductor crystals. These are observed in CuCl nanocrystal excitonic luminescence spectra under resonant size-selective excitation. The theory of exciton coupling with spherical phonons through the Fr\"ohlich and deformation potentials is developed and describes both the size dependence of the polaron binding energy and the degree of the exciton-phonon hybridization near resonance, when the $1P$ and $1S$ exciton level spacing coincides with the one- or two-phonon energy.

Time‐resolved exciton luminescence in GaN grown by metalorganic chemical vapor deposition

Abstract: We report the results of time‐resolved studies on the exciton radiative decay in single‐crystal GaN films grown by metalorganic chemical vapor deposition. Time‐resolved photoluminescence (PL) measurements were performed on the samples at various temperatures from 10 to 320 K. The well‐resolved near‐band‐edge luminescence features associated with free excitons and bound excitons in the GaN allow us to unambiguously determine their decay times. We found that the nonradiative recombination processes play an important role and dominate the decay of exciton population. The processes depend on the density of defects and impurities in the GaN samples.

Hidden structure of the low-energy spectrum of a one-dimensional localized Frenkel exciton.

Abstract: Numerical diagonalization of a one-dimensional (1D) Frenkel exciton Hamiltonian with disorder shows that the low-energy part of the 1D localized exciton spectrum has a hidden structure whose energy scale is determined by the exciton localization length. We propose experimental measurements of two exciton optical transitions in linear molecular aggregates as a method to reveal this structure. We also suggest a formula for the enhancement of spontaneous emission rate of the 1D localized Frenkel exciton adequately allowing for its low-energy density of states.